1. Field
The present invention relates to a transmitter capable of pre-compensating for delays caused by optical fiber chromatic dispersion in a system utilizing an optical orthogonal frequency-division multiplexing (OFDM) technology.
2. Related Art
An orthogonal frequency-division multiplexing (OFDM) system is an FDM system utilizing a digital multi-carrier modulation method. A plurality of orthogonal sub-carriers (also referred to as sub-bands) is used to transmit data. The data is cut into several parallel data streams (or referred to as channels) corresponding to the respective sub-carriers. For each sub-carrier, data transmission is performed with an orthogonal modulation technology with a relatively low symbol rate. In such a manner, compared with a conventional single-carrier, a higher total data rate may be obtained within the same bandwidth.
FIGS. 1A and 1B are schematic views of spectrum distribution comparison of conventional direct transmission and OFDM transmission respectively. The greatest difference between the direct transmission and the OFDM transmission is bandwidth distribution. FIG. 1A shows that bandwidth occupied by the direct transmission is f0. If the bandwidth f0 is subdivided into five equal parts with equal widths in an OFDM manner, and respective sub-bands (that is, the sub-carriers) are orthogonal to one another, new spectrum distribution will be as shown in FIG. 1B. In the OFDM transmission, as long as the sub-bands are enough, basically, frequency response of the band may be approximately considered flat for every sub-band. That is to say, only one equalizer with a single coefficient is needed for each sub-band to overcome attenuation and phase distortion of each sub-channel. In addition, as a data rate of transmission of each sub-band is much lower than the data rate of original direct transmission, and operation clock of the equalizer is surely decreased in equal proportion.
For application of OFDM technology in a field of wireless communication, a common problem is a multi-path effect. The multi-path effect will result in time-spreading and inter-symbol interference (ISI) problem. This is the so-called frequency-selective channel. This frequency-selective problem is usually solved by adding a guard interval into a symbol of each OFDM. This approach will increase a symbol period, and occupy the bandwidth for data transmission.
When the OFDM technology is applied in an optical communication system, as light rays are transmitted in the same optical fiber, the multi-path effect of an optical-OFDM system is not significant. However, due to a phenomenon of optical fiber chromatic dispersion, when a receiving end receives signals, a problem of inter-channel synchronization and an ISI problem similar to the multi-path will occur.
The optical fiber chromatic dispersion causes that a transmission speed of optical signals with higher frequencies is lower than a transmission speed of optical signals with lower frequencies when the optical signals are transmitted through the optical fiber. In the optical-OFDM technology, as each sub-carrier is transmitted at a different frequency, although a transmitter transmits the respective sub-carriers at the same time, the respective sub-carriers received at the receiving end reach at different time. This is the so-called group delay phenomenon.
The details of the group delay phenomenon may be obtained with reference to FIGS. 2A and 2B, and FIGS. 2A and 2B are schematic views of signals transmitted and received at a transmitting end and a receiving end of an optical-OFDM system respectively. FIGS. 2A and 2B are schematic views showing an OFDM signal in a frequency domain. In the drawings, a horizontal axis indicates time, and a vertical axis indicates frequency. Each sub-carrier (sub-band) uses one band. Taking FIG. 2A as an example, each sub-carrier uses a bandwidth of df in the figure. A band (frequency band) of a first sub-carrier is M*df, and the next is (M+1)*df, and the rest may be deduced through analog. It is shown in the figures that each sub-carrier only has data of one symbol. In practical transmission, consecutive symbols are transmitted and received. FIG. 2A is a schematic view of symbols transmitted at the transmitting end. It can be seen from the figure that respective symbols at the transmitting end have the same initial time. As the respective symbols have the same durations, the symbols at the transmitting end also have the same end time. Next, FIG. 2B is a schematic view of signals received at the receiving end after the OFDM signal is transmitted through an optical fiber. It can be seen from the figure that the sub-carriers with lower frequencies (that is, the sub-carriers at the upper part of the frequency axis) are received earlier, while the sub-carriers with higher frequencies (that is, the sub-carriers at the lower part of the frequency axis) reach later. This is the so-called group delay's variation/dispersion.
In order to solve the group delay's variation problem, those in the art add a guard interval into each symbol of a sub-carrier. The guard interval may be a cyclic prefix or a cyclic postfix, and the like. By setting the guard interval, when the receiving end extracts data within the same time interval (that is, the original symbol length added with the guard interval duration), data of each sub-carrier will be complete. Decoding may be performed only upon determining the starting point.
Although the guard interval may solve the group delay's variation problem caused by the optical fiber chromatic dispersion, the more sever optical fiber chromatic dispersion may result in longer guard interval to be added. However, the longer guard interval occupies more bandwidth. That is to say, the bandwidth that can be used to transmit data is reduced. The group delay generated by the chromatic dispersion is in direct proportion to a transmission distance in the optical fiber and a frequency difference between sub-carriers. That is to say, the larger total bandwidth or the longer transmission distance results in the greater group delay's variation, and thus the longer guard interval is required to solve the problem.
An essay on application of the OFDM technology in a field of optical communication is one published by W. Shieh, X. Yi and Y Tang (referring to “Experimental Demonstration of Transmission of Coherent Optical-OFDM Systems”, Optical Fiber Communications Conference (OFC) 07, OMP2, March, 2007). The assay discloses experimental demonstration of a coherent optical-OFDM system. A situation that 128 OFDM sub-carriers using a transmission speed of 8 Gb/s (8×109 bits/second) are transmitted and received through an optical fiber of 1000 kilometers (km) is shown. In the essay, problems generated by optical fiber chromatic dispersion are overcome in the manner of guard intervals.
Usually the guard interval added due to the optical fiber chromatic dispersion occupies at least 6% of the communication bandwidth. If the bandwidth occupied to serve as pilot carriers (scatter pilot, preamble or mid-amble) for channel response estimation and the bandwidth of control signals are subtracted from the communication bandwidth, the bandwidth for data transmission will be further reduced.